Solar boilers which are heated by concentrating the sun's rays by reflecting them from heliostats are known in the art. Central tower solar energy systems in which a plurality of heliostat mirrors reflect the sun's rays onto a solar receiver mounted atop a tower are described in U.S. Pat. Nos. 3,892,433; 3,924,604; and 4,034,735. Heliostats are described in U.S. Pat. Nos. 4,122,827; 4,209,236; 4,440,150; 4,536,847; 4,564,275; 5,929,530; and 6,959,993.